1. Field of the Invention
The present invention relates to a yellow toner for use in image formation employing so-called electrophotography, including electrostatic copiers and laser beam printers, and a developer containing the toner, and an image forming method using the toner.
2. Description of the Related Art
Conventionally, in electrophotographic apparatuses and electrostatic image recording apparatuses, an electric or magnetic latent image is visualized using a toner. For example, in electrophotography, a latent electrostatic image is formed on a photoconductor, and then the latent electrostatic image is developed using a toner to thereby form a toner image. The toner image is usually transferred onto a transfer material such as paper, and then fixed on the transfer material by a method such as heating.
A method for producing a toner used for developing a latent electrostatic image is broadly classified into a pulverization method and a polymerization method.
In the pulverization method, a thermoplastic resin, a colorant, a charge control agent, an offset preventing agent and the like are melted and mixed so as to form a uniform dispersion, and the resultant composition is pulverized and classified to thereby produce a toner. According to the pulverization method, a toner having excellent properties to a certain degree can be produced, however, the particle size distribution thereof tends to be wider. In order to obtain a copy image having a high resolution and a high gradation, for example, it is necessary to remove fine powder particles each having a diameter of 5 μm or smaller, particularly 3 μm or smaller, and coarse powder particles each having a diameter of 20 μm or greater, and thus the pulverization method has a disadvantage in that the yield becomes very low. In addition, in the pulverization method, it is difficult to uniformly disperse a releasing agent, a pigment and the like in a thermoplastic resin. Moreover, the pulverization method has such a problem that the pigment is exposed to a surface of a toner due to pulverization, and the surface of the toner is unevenly charged, and charge distribution on the toner becomes broad, causing degradation in developing properties. Therefore, at present, the pulverization method cannot sufficiently respond to superior performances demanded for a toner.
On the other hand, in the polymerization method, a pulverization step and a kneading step can be omitted, thereby contributing greatly to cost saving, such as energy saving, shortening of production time, improvement of step yield, and the like. Moreover, by the polymerization method, the size of a toner particle is made smaller, and sharp particle size distribution can be easily obtained, compared to the pulverization method. Furthermore, the polymerization method greatly contributes to improvement of image quality. Thus, the polymerization method is a promising production method.
In electrophotography, there are demands for improvement of quality of full color images. To achieve the improvement of image quality, it is necessary for each toner to have a wide color reproduction range.
Moreover, to achieve the improvement of image quality without causing any problem, it is necessary to highly disperse, in a toner, a colorant having excellent transparency, light resistance, and heat resistance.
However, there are many problems in a colorant for yellow toner, which is required for full color image formation. For example, generally a dye is excellent in transparency, but poor in light resistance, causing problems in storage stability of an image. A pigment is superior in light resistance to such a dye. However, the light resistance of a pigment for yellow toner is not sufficient, compared to a copper phthalocyanine pigment used as a pigment for magenta toner.
Therefore, disazo compounds having excellent light resistance have been proposed (Japanese Patent Application Publication (JP-B) No. 02-37949). The proposed disazo compounds are a group of compounds, which are typified by C. I. PIGMENT YELLOW 180, and one of azo pigments having excellent light resistance, and heat resistance, and satisfying the recent demands for safety.
Moreover, a yellow toner using C. I. PIGMENT YELLOW 180 has been proposed (see Japanese Patent Application Laid-Open (JP-A) Nos. 06-230607, 06-266163, and 08-262799). However, in these proposed techniques, coloring performance is low, and transparency is far from excellent.
There is a proposal of a toner produced by a method of dispersing an oil phase in an aqueous phase using C. I. PIGMENT YELLOW 180, such as an emulsification dispersion method (JP-A No. 11-65172), or a suspension polymerization. However, since C. I. PIGMENT YELLOW 180 has relatively high hydrophilicity, there are such problems in these proposed techniques as segregation of C. I. PIGMENT YELLOW 180 on particle surfaces, and reaggregation inside the particles, causing difficulty in using C. I. PIGMENT YELLOW 180.
There is a proposal of a toner using C. I. PIGMENT YELLOW 185 of an isoindoline pigment (see JP-A No. 2005-106932). C. I. PIGMENT YELLOW 185 exhibits bright yellow color, involves less color fading, and is excellent in light resistance. However, C. I. PIGMENT YELLOW 185 has an absorption at 500 nm to 600 nm in a spectrum, thus C. I. PIGMENT YELLOW 185 is a yellow pigment strongly tinged with blue, and yellow color reproducibility is poor. For example, a* is largely shifted to a negative side on a*-b*coordinates in accordance with Japan Color which is printing color standards for offset printing.
Thus, a yellow toner having excellent transparency, light resistance, color reproducibility, such as chroma, hue, etc. and high coloring performance, and causing less background smear in long-term running at high temperature and high humidity has not been obtained. At present, there are demands for a yellow toner having excellent transparency, light resistance, and color reproducibility, such as chroma, hue, etc. and high coloring performance, and causing less background smear in long-term running at high temperature and high humidity, and a developer containing the toner, and an image forming method using the toner.